The instant invention is a one-step process for producing polydiorganosiloxanes (siloxanes). In the described process, a mixture comprising diorganodichlorosilane and a source of triorganosilyl groups is contacted with excess water essentially saturated with hydrogen chloride. Product siloxanes are isolated as well as saturated aqueous hydrogen chloride and anhydrous hydrogen chloride. The amount of triorganosilyl groups present in the mixture is controlled to set the degree of polymerization of product siloxanes. The isolated saturated aqueous hydrogen chloride is recycled to the process. Excess chloride generated in the process is collected as anhydrous hydrogen chloride.
Current production of polydiorganosiloxane polymers, for example, polydimethylsiloxane, is a multi-step process. In a typical process, a first step involves hydrolyzing dimethyldichlorosilane in excess water. The hydrolysate rapidly undergoes condensation to form an equilibrium mixture of low molecular weight permethylcyclosiloxanes and low molecular weight dimethylsiloxane linears, with end-terminal hydroxy substitution. A by-product of this reaction is an aqueous hydrogen chloride solution.
In a second step, the low molecular weight condensation products are then further polymerized and end blocked in an acid or base catalyzed process to create high molecular weight polydimethylsiloxane fluids.
The aqueous hydrogen chloride produced in the first hydrolysis step presents a problem of acid disposal or recovery. Both the expense of disposing of the environmentally hazardous aqueous hydrogen chloride solution and the inherent value of chloride makes recovery a preferred option. One procedure for recovering anhydrous hydrogen chloride from the aqueous hydrogen chloride is to distill the solution to produce a constant boiling hydrogen chloride-water azeotrope along with anhydrous hydrogen chloride. A significant amount of energy is required in this process. Therefore, one objective of the instant process is to provide a method for efficient recovery of hydrogen chloride. This objective is achieved by running the present process utilizing a saturated aqueous hydrogen chloride solution, thereby causing additional hydrogen chloride liberated during the hydrolysis reaction to evolve as anhydrous hydrogen chloride gas. This negates the need for a distillation step to recover hydrogen chloride generated by the process.
Hyde et al. U.S. Pat. No. 2,779,776 teaches that the reaction between a siloxane and an aqueous acid is reversible and that the degree of polymerization of the siloxane at the point of equilibrium of the reversible reaction is determined by the concentration of the acid in the aqueous phase. Since the instant process is run under conditions of essentially saturated acid, to allow flexibility of the process, an alternative method of controlling polymer length is desirable. Therefore, a second objective of the instant invention is to provide a process whereby the degree of polymerization is independent of acid concentration.
Hansen, et al., Co-Pending U.S. Pat. application Ser. No. 07/612,655, teaches that in the acid catalyzed condensation of polydiorganosiloxanes containing end-terminal hydroxy functionality, control of polymer length can be effected by the presence of a source of triorganosilyl groups. This process is taught to be much faster than the equilibrium process, as exemplified by Hyde, supra, and therefore the principle determinate of polymer length.
Wilcock, U.S. Pat. No. 2,491,843, teaches a method of contacting an aqueous concentrated hydrochloric acid solution and a mixture of trimethylchlorosilane and methyldichlorosilane to form a plurality of linear polymethylhydrogensiloxanes end blocked with trimethylsilyl groups. Wilcock does not appear to have recognized the importance of controlling the level of trimethylchlorosilane to dictate polymer chain length and makes no provision for assuring evolution of gaseous hydrogen chloride.
Schwenker, U.S. Pat. No. 2,758,124, teaches a continuous process for preparing polyorganosiloxanes. The process comprises: (1) simultaneously passing a mixture of an organochlorosilane and water which may contain up to about 32 percent, by weight hydrogen chloride (based on the total weight of water and hydrogen chloride) into a circulating system: (2) continuing the introduction of acid-free or acid-containing water and organochlorosilane until partial overflow of the formed polyorganosiloxane and acid-containing water of greater hydrogen chloride concentration (than the original feed comprised) is effected: (3) removing said overflow materials and separating the formed polyorganosiloxane from the acid-containing water, while at the same time recycling the remaining high-acid content water and residual polyorganosiloxane separated from the overflow, so as to diffuse the same into the incoming feed of water or lower acid-containing water and organochlorosilane. The process is run under conditions to substantially repress evolution of gaseous hydrogen chloride.
Hajjar. U.S. Pat. No. 4,609,751, describes a method for hydrolyzing chlorosilanes, for example. dimethyldichlorosilane, to produce polydimethylsiloxane hydrolyzate and an aqueous solution of hydrogen chloride along with anhydrous hydrogen chloride. Chlorosilane hydrolysis is effected in the presence of a substantially stoichiometric equivalence of water which results in the direct generation of anhydrous hydrogen chloride and a saturated aqueous hydrogen chloride. The saturated aqueous hydrogen chloride can be recycled to the chlorosilane hydrolysis step.
The hydrolysis of chlorosilanes occurs rapidly upon contact with water. Therefore limiting the availability of water, as described by Hajjar, supra, has the undesirable effect of limiting the rate of the hydrolysis reaction and subsequent condensation reaction. Therefore, it is a third objective of the instant invention to provide a process where water is present in stoichiometric excess in relation to chloride present as chlorosilane.
Polydiorganosiloxanes prepared by the method of the instant invention are useful in applications such as those for which polydiorganosiloxanes prepared by standard methods are used. These uses include, for example, lubricants, heat transfer media, damping fluids, coatings for glass and ceramics, release agents, and additives into other chemical formulations.